This invention relates generally to rocket propulsion systems and, more particularly, to rocket propulsion systems for placing and maintaining spacecraft in planetary orbits. Although the invention has broad application to unmanned spacecraft, it is particularly concerned with launching and maintaining satellites in geosynchronous orbits. Placing a geosynchronous satellite into orbit typically involves three principal mission phases. First the satellite is placed in low earth orbit not far above the earth's atmosphere, either as part of the payload of a space shuttle vehicle or on a conventional non-resusable rocket vehicle. In the second phase, the satellite orbit has its apogee or highest point raised in altitude by one or more rocket "burns" at a selected point in the orbit, until the apogee is approximately at geosynchronous altitude. Finally, the satellite is given an apogee "kick," i.e. a further rocket burn at apogee that circularizes the orbit at geosynchronous altitude.
Once in orbit, rocket engines will be called on for two further functions: station keeping and attitude control, which are sometimes referred to collectively as reaction control system (RCS) functions. Satellites are usually required to maintain a particular "station" with respect to the earth's surface. Maintaining this station requires the expenditure of energy, even though the orbit is theoretically self-sustaining and geosynchronous. Various factors, such as the non-spherical nature of the earth, the gravitational influences of the moon and sun, and so forth, require that the orbit be corrected from time to time if the required station is to be maintained. Attitude control is simply the use of multiple rocket engines on the spacecraft to maintain a particular angular attitude of the vehicle. This may be needed, for example, to point an antenna or other sensor at the earth, the sun, or a star.
Rocket engines associated with orbiting spacecraft may be called upon to perform the various functions of orbital transfer, station keeping and attitude control. Unfortunately, the performance characteristics required for these functions are not identical. A figure of merit often used in the comparison of rocket engines is the specific impulse, I.sub.sp, which is defined as the thrust developed by the engine per unit of propellant weight flow rate. If the thrust is measured in pounds and the flow rate in pounds per second, the units for the measurement of specific impulse are seconds. The specific impulse is analogous to a miles-per-gallon figure for an automobile, since it measures how much thrust is developed for a unit fuel flow rate.
Another measure of performance is, of course, the thrust force generated by the engine. For the rapid acceleration that is required in a transition to geosynchronous orbit, particularly at the apogee "kick" phase of a mission, an engine with a relatively large thrust is required, perhaps generating up to several thousand pounds of thrust force. The specific impulse is also important, and should be in the 300-400 second range. For station keeping and attitude control, high thrust is not quite so important, since most station-keeping and attitude control maneuvers can be accomplished with low-thrust burns of the rocket engines. However, fuel economy is very important for these activities if the vehicle is to be sustained in orbit for a prolonged period.
Typically, the approaches followed to date have involved using multiple fuels and engine systems for the two tasks. For example, a solid rocket is used for the apogee kick engine and hydrazine catalytic engines for the station keeping and attitude control system thrusters. There is nothing inherently wrong with this traditional approach, except that the use of two separate propulsion systems severely limits the size of the useful payload that can be placed and maintained in orbit.
Some improvement can be obtained using an integrated bipropellant system, in which both the apogee kick engine and the RCS thrusters use a bipropellant fuel system, such as monomethyl hydrazine (MMH), as the fuel and nitrogen tetroxide (N.sub.2 O.sub.4) as the oxidizer. However, there is still room for further improvement in the payload that can be placed in orbit for a given mission. Another way to look at the problem is that there is room for improvement in the lifetime that a given spacecraft payload may be maintained in orbit. With a more efficient propulsion system, a greater payload may be maintained in orbit for a given time, or the same payload may be maintained in orbit for a longer time.
The present invention provides a more efficient propulsion system suitable for geosynchronous and other high-energy mission spacecraft programs.